1. Field of the Invention
The present invention relates to an optical information recording and/or reproducing apparatus and, more particularly, to an optical information recording and/or reproducing apparatus for recording information or an optical information recording medium and/or reproducing the information recorded on the optical information recording medium and/or erasing the information recorded on the optical information recording medium by relatively moving a light spot to tracks on the optical information recording medium while tracking and/or focusing the light spot relative to a track.
2. Related Background Art
Hitherto, various kinds of disk-shaped mediums, card-shaped mediums, tape-shaped mediums, and the like have been known as forms of optical information recording media for recording information by using light or for reading out the recorded information. Among them, in the case of an optical information recording medium in the form of a card (hereinafter, referred, to as an "optical card"), a large demand is expected, since the card is a small and light-weight information recording medium having a large recording capacity which is convenient to carry.
FIG. 5 is a schematic plan view for explaining a construction of the optical card.
In the diagram, an information recording area is provided in an optical card 101. Track selection areas 104a and 104b are provided in both edge portions of the area 102. Information tracks 103 are provided in parts of the information recording area 102 and track selection areas 104a and 104b. Information is recorded on the information tracks 103. Reference numeral 105 indicates a home position of a light spot.
Information is recorded on the optical card 101 as an optically detectable recording pit train (information track) by scanning the information track by a light beam modulated in accordance with recording information and converged to a microspot.
In this case, to accurately record the information without causing any trouble such as intersection of information tracks or the like, the irradiating position of the light spot needs to be controlled in the direction perpendicular to the scanning direction in the optical card surface (auto tracking, hereinafter, referred to as "AT"). On the other hand, to irradiate the light beam as a microspot of a stable size irrespective of any bending or mechanical errors of the optical card, the light spot needs to be controlled in the direction perpendicular to the optical card surface (auto focusing, hereinafter, referred to as "AF"). In addition, the above AT and AF are also necessary for reproduction.
The AT operation will now be simply explained hereinbelow. The detailed description of the AF operation will be made later.
Although various kinds of methods for AF are known, an astigmatism method and a knife edge method can be mentioned as two typical methods among them.
The astigmatism method is a method of detecting a focusing error by using an optical part for generating astigmatism and has been disclosed in detail in JP-B-53-39123.
On the other hand, the knife, edge method is a method whereby a knife edge is arranged to correspond a width of an optical passage of the converged light returned from a recording medium and a focusing error is detected by detecting a movement amount of a light spot image on a photo-sensitive element.
Hitherto, to execute such an AF operation, there has been performed a pull-in operation such that an objective lens is first moved in the, focusing servo OFF state until the light spot reaches an in-focus state on the recording surface of an optical card, and after the in-focus state is obtained, the focusing servo is made operative. (In general, before the start of the recording and reproducing operations of information, the objective lens is held in an initial state in which its focal point deviates from the recording surface of the optical card.)
Since a dynamic range of a focusing error signal is small enough to be tens of .mu.m, upon execution of the AF, the pull-in operation is necessary to pull in the focusing servo within the dynamic range.
The in-focus state can be obtained in a manner such that the light reflected (or transmitted) from a recording pit train on the optical card is detected by a photoelectric conversion element, an electric focusing signal from the photoelectric conversion element is amplified to obtain a voltage value (V), a relative position of an objective lens from the in-focus state is determined on the basis of the voltage value, and the objective lens is moved on the basis of the relative position.
FIG. 6A is a characteristic graph showing the relative distance between an optical card and a light spot in a case when the in-focus position is used as a reference. FIG. 6B is a characteristic graph showing a change in the voltage value which is obtained by amplifying an electric focusing signal upon a normal operation. As shown by a broken line in FIG. 6A, when the objective lens is moved near the in-focus state, as shown in FIG. 6B, the voltage obtained by amplifying the electric focusing signal changes in the form of an S-character shape (what is called an S-shaped curve) (time t.sub.11 to time t.sub.16).
Hitherto, to detect the in-focus state and to pull in the servo, a servo loop is closed when the voltage value V is set to 0 V (a zero-cross level or a zero-cross point) (at time t.sub.15).
However, in the conventional AF operation, there is a drawback such that the servo loop is closed even when the, voltage value V is set to 0 V because of the objective lens being away from the in-focus position due to the occurrence of a vibration or electric noises. Such a state will now be described hereinbelow with reference to FIG. 7.
FIG. 7 is a characteristic graph showing a change in the voltage value obtained by amplifying the electric focusing signal in a case when a vibration has occurred.
As shown in FIG. 7, when the objective lens is moved toward the in-focus position from time t.sub.10, the voltage value V is set to 0 V until time t.sub.11 and is set to the negative maximum value at time t.sub.12 and rises after that.
When a vibration occurs at time t.sub.13 and the distance between the optical card and the light spot changes in such a direction as to be away from the in-focus position as shown by a solid line in FIG. 6A, the voltage value V is set to the same value of 0 V at time t.sub.14 as that at time t.sub.11 as shown in FIG. 7. In the conventional AF operation, the servo loop is closed at that time point. However, since the light spot is away from the in-focus position in such a state, the pull-in operation of the servo fails.
The reason why the servo loop is not closed, although the voltage value V is held to 0 V for the time interval from t.sub.10 to t.sub.11 in FIG. 7, is because 0 V, which is detected after the voltage value V has exceeded a predetermined threshold value different from 0 V after the start of the pull-in operation, is used to decide the in-focus position.